Find characters between double quotes “

$string = '"LastName, FirstName" ([samaccountname])'
[Regex]::Matches($string, '(?<=\")(.*?)(?=\")').Value

Details

Uses a regex expression with find all characters that match the expression.

Example

PS C:\> $string = '"LastName, FirstName" ([samaccountname])'
>> [Regex]::Matches($string, '(?<=\")(.*?)(?=\")').Value

LastName, FirstName